Volvo 20 20 and 3 blade propeller

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chwarae

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Had a sea trial on a gibsea 33 with a Volvo 20 /20 engine. The engine would not go above 2000 revs in gear although the engine ran way above those revs in neutral. There is a three blade prop on the boat but looking at the Volvo website it recommends 3 blade from 25hp up. The 20/ 20 only being 18hp and Volvo recommend a 2 blade prop for this horsepower. Incidentally the boat would not go above 5 knots and most of the time 4 knots under engine.
Thoughts please.
 
Number of blades is irrelevant but has the prop been recently changed?
Max revs is way down should be over 3000
Do you have any black smoke ?
There are various factors that are responsible for lack of revs but usually it boils down to excess drag weight lack of fuel or the topical choked exhaust
 
No black smoke, engine starting fine.fuel not a problem. Engine runs fine but will not go above 2000 revs in gear. Prop a 3 blade folding volvo. Prop replaced some years ago. Owner states that boat does 5.5 knots at 2000 revs. Boat was only doing 4 knots maybe slightly against tide. Boat has a 28 foot waterline. I suspect wrongly propped and some growth on prop. Or prop suffering from wear. Thanks for your comments. Any other thoughts please?
 
You obviously have some doubts regarding the prop but do not rush down that road
Overload produces unburnt fuel which produces smoke
If there is no smoke then the engine is not being overloaded so choked exhaust is an increased likelihood
The previous owner show say what the max rev have been
Not running a diesel too hard is sparing the rod to spoil the child
Lack of fuel caused by dirty or blocked supply will also give lack of power
 
Whatever is causing it it's a problem. You should get more speed than that with that engine and that boat. Too big a prop may cause a drop in revs and some owners do that for more comfortable cruising but I would think that you should still get a decent speed through the water. You don't.
It's something you would have to get sorted out before going any further with the purchase. Lifts in and out to get a prop correct can be costly so tell the owner your concerns and let him sort and don't go any further until it's fixed.
If it's not a dirty hull and prop, best case would be a blocked elbow.
 
Measuring speed by GPS. Logs can be + or - according to state of log. As stated engine out of gear plenty of revs but not over 2000 when in gear. Don't think it's an engine issue but something is not right. Thought 28 ft. waterline would provide comfortable 6, 6.5 knots under engine. Again thanks for comments.
 
I have a Gib'sea 98 with a LWL of 8.30 meters and get 6.5 knots from a Volvo Penta 2002 (18hp), at 2500 revs it will turn the prop faster but with no significant increase on speed through the water. Different engine but "similar" hull you will have a longer LWL so should get over 6.5 knots.
 
chwarae said:
Measuring speed by GPS. Logs can be + or - according to state of log.

GPS does not measure speed through water but speed over land needs tidal and wind correction to get speed through water





As stated engine out of gear plenty of revs but not over 2000 when in gear. Don't think it's an engine issue but something is not right. Thought 28 ft. waterline would provide comfortable 6, 6.5 knots under engine. Again thanks for comments.
Click to expand...
 
Just an update.hull and prop cleaned today. What a difference, over 3200 revs over 7 knots. 2500 revs 6knots. An awful lot of crud on prop. After a good clean a remarkable difference. Thanks for everyone's advice.
 
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